linux-kernel - Re: [PATCH v2] PM: runtime: Return properly from rpm_resume() if dev->power.needs_force

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Message-ID: <75366bfac9fcd4f8c35309193705f0277a164ae4.camel@nxp.com>
Date:   Fri, 23 Sep 2022 23:22:35 +0800
From:   Liu Ying <victor.liu@....com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-imx@....com,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v2] PM: runtime: Return properly from rpm_resume() if
 dev->power.needs_force_resume flag is set

On Fri, 2022-09-23 at 15:48 +0200, Ulf Hansson wrote:
> On Fri, 23 Sept 2022 at 14:47, Liu Ying <victor.liu@....com> wrote:
> > 
> > After a device transitions to sleep state through it's system
> > suspend
> > callback pm_runtime_force_suspend(), the device's driver may still
> > try
> > to do runtime PM for the device(runtime suspend first and then
> > runtime
> > resume) although runtime PM is disabled by that callback.  The
> > runtime
> > PM operations would not touch the device effectively and the device
> > is
> > assumed to be resumed through it's system resume callback
> > pm_runtime_force_resume().
> 
> This sounds like a fragile use case to me. In principle you want to
> allow the device to be runtime resumed/suspended, after the device
> has
> already been put into a low power state through the regular system
> suspend callback. Normally it seems better to prevent this from
> happening, completely.

Not sure if we really may prevent this from happening completely.

> 
> That said, in this case, I wonder if a better option would be to
> point
> ->suspend_late() to pm_runtime_force_suspend() and ->resume_early()
> to
> pm_runtime_force_resume(), rather than using the regular
> ->suspend|resume() callbacks. This should avoid the problem, I think,
> no?

I thought about this and it actually works for my particular
panel-simple case.  What worries me is that the device(DRM device in my
case) which triggers the runtime PM operations may also use            
->suspend_late/resume_early() callbacks for whatever reasons, hence no
fixed order to suspend/resume the two devices(like panel device and DRM
device).

Also, not sure if there is any sequence issue by using the            
->suspend_late/resume_early() callbacks in the panel-simple driver,
since it's written for quite a few display panels which may work with
various DRM devices - don't want to break any of them.

Regards,
Liu Ying

> 
> Note that, the PM core also disables runtime PM for the device in
> __device_suspend_late(). For good reasons.
> 
> [...]
> 
> Kind regards
> Uffe